Method for solid oxide fuel cell fabrication

ABSTRACT

A method of making a solid oxide fuel cell (SOFC) includes forming a first sublayer of a first electrode on a first side of a planar solid oxide electrolyte and drying the first sublayer of the first electrode. The method also includes forming a second sublayer of the first electrode on the dried first sublayer of the first electrode prior to firing the first sublayer of the first electrode, firing the first and second sublayers of the first electrode during the same first firing step, and forming a second electrode on a second side of the solid oxide electrolyte.

RELATED APPLICATION

This application claims priority to U.S. Provisional Application61/374,424, filed on Aug. 17, 2010.

BACKGROUND OF THE INVENTION

The present invention is directed to fuel cell components generally andtowards fabrication of solid oxide fuel cell anode and cathodematerials.

Fuel cells are electrochemical devices which can convert energy storedin fuels to electrical energy with high efficiencies. Electrolyzer cellsare electrochemical devices which can use electrical energy to reduce agiven material, such as water, to generate a fuel, such as hydrogen. Thefuel and electrolyzer cells may comprise reversible cells which operatein both fuel cell and electrolysis mode.

In a high temperature fuel cell system, such as a solid oxide fuel cell(SOFC) system, an oxidizing flow is passed through the cathode side ofthe fuel cell while a fuel flow is passed through the anode side of thefuel cell. The oxidizing flow is typically air, while the fuel flow canbe a hydrocarbon fuel, such as methane, natural gas, propane, pentane,ethanol, or methanol. The fuel cell, operating at a typical temperaturebetween 750° C. and 950° C., enables the transport of negatively chargedoxygen ions from the cathode flow stream to the anode flow stream, wherethe ion combines with either free hydrogen or hydrogen in a hydrocarbonmolecule to form water vapor and/or with carbon monoxide to form carbondioxide. The excess electrons from the negatively charged ion are routedback to the cathode side of the fuel cell through an electrical circuitcompleted between anode and cathode, resulting in an electrical currentflow through the circuit. A solid oxide reversible fuel cell (SORFC)system generates electrical energy and reactant product (i.e., oxidizedfuel) from fuel and oxidizer in a fuel cell or discharge mode andgenerates the fuel and oxidant using electrical energy in anelectrolysis or charge mode.

Anode electrodes operating under conditions of extreme fuel starvationare usually irreversibly damaged. Such starvation conditions are usuallyencountered in stacks where isolated repeat elements (i.e., specificfuel cells) obtain less fuel than their neighboring elements (i.e., theneighboring fuel cells). These elements witness effective fuelutilization in excess of 100%. Similar conditions may arise duringsystem transitions or operating anomalies where the fuel supply to thecell does not correspond to the current drawn. Under thesecircumstances, the oxygen ion flux to the anode will oxidize the anodeconstituents. Nickel present at the three phase boundary of traditionalanodes will instantaneously oxidize. The phase change from Ni metal toNiO is accompanied by a change in volume that causes mechanical damageat the anode/electrolyte interface. This mechanical damage ischaracterized by delamination of the electrode from the electrolytewhich increases the specific resistance of the cell and dramaticallydecreases the stack performance. To avoid oxidation of the nickel andmechanical damage of the electrode electrolyte interface, which leads todelamination, one prior art solution was to employ an all ceramic anode.While the ceramic anodes show better stability in starvation conditions,they are associated with high polarization losses.

Solid oxide fuel cells operate using hydrocarbon based fuel. SOFCoperate in one of two modes; a pre-reforming mode or an internallyreforming mode. In the pre-reforming mode, the hydrocarbon fuel ispre-reformed into a syn-gas (CO+H₂) before entering the fuel cell. Theanode provides an electro-catalytically active surface for oxidation ofthe pre-reformed fuel and ensures sufficient oxide-ionic and electronicconduction. In the internally reforming mode, the hydrocarbon fuelenters the solid oxide fuel cell where it is exposed to the anode. As inthe pre-reforming mode, the anode provides both fuel oxidation and ionicand electronic transport. However, the anode must also internally reformthe hydrocarbon fuel. State-of-the-art anodes are composites. Theseanodes are composed of an electrocatalytic material that is primarily anelectronic conductor, such as Ni metal, and an oxide-ionic conductivematerial. Traditionally, state of the art anodes are Ni-ceria andNi-zirconia. These anodes operating under internal reforming mode aresusceptible to failure by anode delamination, structural failure at theleading edge where the hydrocarbon fuel enters the cell, or nickeldusting from internal Ni-carbide formation resulting in embrittlement.

SUMMARY

A method of making a solid oxide fuel cell (SOFC) includes forming afirst sublayer of a first electrode on a first side of a planar solidoxide electrolyte and drying the first sublayer of the first electrode.The method also includes forming a second sublayer of the firstelectrode on the dried first sublayer of the first electrode prior tofiring the first sublayer of the first electrode, firing the first andsecond sublayers of the first electrode during the same first firingstep, and forming a second electrode on a second side of the solid oxideelectrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate side cross-sectional views and FIG. 3illustrates a top view of SOFCs of the embodiments of the invention.

FIG. 4 illustrates a side cross sectional view of SOFC stack of anembodiment of the invention.

FIGS. 5A and 5B depict SEM images of anode electrode sub-layers in asolid oxide fuel. FIG. 5A shows an anode of a comparative example. FIG.5B illustrates an anode of an example of the invention.

FIG. 6 is a plot of mechanical load testing data for cells according toa comparative example and an example of the invention.

FIG. 7 is a plot of the beginning of life median output voltage for acompilation of several stacks while varying temperature for stacksaccording to a comparative example and example of the invention.

FIG. 8 is a plot of cell potential after 200 hours of operation for a 25cell stack containing cells according, to as comparative example(un-circled) and an example of the invention (circled).

FIG. 9 is a graph of methane conversion versus mass flow for pure nickelof the comparative example and for a Ni—Co alloy of the example of theinvention at 750 and 800° C.

FIG. 10 is schematic representation of the apparatus and method stepsused to fabricate a SOFC cell of one embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In one embodiment of the invention, a SOFC anode electrode is formed byscreen printing two or more anode sublayers. These sublayers are printedseparately and allowed to dry at a low temperature and then they arefired in a single anode layer firing. Thus, in this embodiment, ratherthan sequentially depositing (e.g., printing) and firing each anodesublayer before depositing the next anode sublayer, the method of thisembodiment includes depositing (e.g., screen printing) a first anodesublayer over the electrolyte, allowing the first sublayer to dry atrelatively low temperature, and then depositing (e.g., screen printing)a second anode sublayer on top of the dried but unfired first anodesublayer. After all or a desired number of anode sublayers are depositedand dried, the plural anode sublayers are fired together in one firingstep and a temperature that is at least three times higher than thedrying temperature. For example, the drying steps may be conducted at atemperature of less than 150 C, such as 50 to 100 C, for example 70 to80 C. The firing may be conducted at a temperature greater than 1000 C,such as 1100 C to 1400 C.

In another embodiment of the invention, a cathode electrode is formed byscreen printing two or more cathode sublayers. These sublayers areprinted separately and allowed to dry at a low temperature and then theyare fired in a single cathode layer firing. Thus, in this embodiment,rather than sequentially depositing (e.g., printing) and firing eachcathode sublayer before depositing the next cathode sublayer, the methodof this embodiment includes depositing (e.g., screen printing) a firstcathode sublayer over the electrolyte, allowing the first sublayer todry at relatively low temperature, and then depositing (e.g., screenprinting) a second cathode sublayer on top of the dried but unfiredfirst cathode sublayer. After all or a desired number of cathodesublayers are deposited and dried, the plural cathode sublayers arefired together in one firing step and a temperature that is at leastthree times higher than the drying temperature. For example, the dryingsteps may be conducted at a temperature of less than 150 C, such as 50to 100 C, for example 70 to 80 C. The firing may be conducted at atemperature greater than 1000 C, such as 1100 C to 1400 C.

In another embodiment of the invention, both the anode and the cathodeelectrode are formed by screen printing two or more anode and two ormore cathode sublayers. These sublayers are printed separately andallowed to dry at a low temperature and then the anode sublayers arefired in a single anode layer firing and the cathode sublayers are firedin a single cathode layer firing. Preferably, the anode and the cathodelayer firing steps are separate firing steps. Thus, either the anode orcathode sublayers are first deposited, dried and then fired together.Then, the other ones of anode or cathode sublayers are deposited, driedand then fired together. Either the anode or cathode sublayers may bedeposited, dried and fired first.

In an alternative embodiment, the anode and the cathode layer firing isperformed in the same step. In this case, the electrolyte containingdried anode and cathode electrode layers is provided into a furnace andthe anode and cathode are co-fired during the same step.

In the above embodiments, screen printing is a preferred sublayerdeposition method. For example, the screen printing may be performedwith calendered mesh with a high wire density. In the screen printingmethods, the anode and cathode ink preferably are self-leveling and havea relatively high solids loading of 80 to 93 weight percent. This inkformulation and electro-catalyst particle morphology that ischaracterized by a good rheology with a high solids loading allows forvery thin sublayers (i.e., screen prints) containing highly activeelectro-catalysts. When these sublayers are sintered to the solid oxideelectrolyte, they form dense electrodes, e.g., a dense anode electrodewith limited residual strain.

In another embodiment of the invention, an anode electrode for a solidoxide fuel cell allows for the direct internal reforming of hydrocarbonfuels on the anode and reliable operation under fuel starvationconditions. The internal reforming anode will eliminate the need for apre-reformer or an external reformer, thus significantly reducing thecost. The solid oxide fuel cell (SOFC) comprises a cathode electrode, asolid oxide electrolyte, and an anode electrode comprising a firstportion and a second portion, such that the first portion is locatedbetween the electrolyte and the second portion. The anode electrodecomprises a cermet comprising a nickel containing phase and a ceramicphase. The first portion of the anode electrode is a cermet comprising anickel containing phase and a ceramic phase with a lower porosity and alower ratio of the nickel containing phase to the ceramic phase than thesecond portion of the anode electrode. The two portions of the anodeelectrode may be formed from separately deposited and separately driedsublayers which are then fired together as described in the previousembodiments.

In one embodiment, the second portion further comprises a nickelcontaining phase in which the highly electrocatalytically active nickelis substituted in part by a metal which has a lower electrocatalyticactivity than nickel (including non-electrocatalytic metals). The metalmay comprise cobalt (Co) and/or copper (Cu) which is preferably but notnecessarily alloyed with nickel to decrease the catalytic activity ofthe nickel containing phase. Decreased catalytic activity results inlower thermo-mechanical stress, which the inventors believe leads tolower anode delamination and mechanical damage. The substituted nickelcermet, such as a nickel alloy cermet, for example a Ni—Co alloy cermet,also exhibits a lower electrocatalytic activity in comparison with thepure Ni cermet where all other parameters are kept constant.

The embodiments of the invention provide anode electrodes for solidoxide fuel cells, such as reversible SOFCs (i.e., SORFC) andnon-reversible SOFCs, that do not irreversibly deteriorate underconditions of extreme fuel starvation. The embodiments of the inventionconduct internal reformation of hydrocarbon based fuels withoutmechanical damage to the anode. The anode electrodes display improvedoutput efficiency and polarization losses comparable to prior art Ni—YSZanodes. Therefore, the anode conducts fuel oxidation, ionic andelectronic transport, and reforming of the hydrocarbon fuel underconditions of fuel starvation. Furthermore, after a starvation event,the performance of the anode electrodes of the embodiments of theinvention is hardly affected and there is minimal mechanicaldeterioration of the anode.

The anode electrode of one embodiment of the invention is a cermetcomprising a nickel containing phase (i.e., a metal phase which includesnickel) and a ceramic phase. The nickel containing phase preferablycontains nickel in a reduced state, or nickel with one or moreadditional metals, such as cobalt and/or copper, in a reduced state.This phase forms a metal oxide when it is in an oxidized state. Thus,the anode electrode is preferably annealed in a reducing atmosphereprior to operation to reduce the nickel oxide to nickel. The nickelcontaining phase may consist essentially of only nickel or it mayinclude other metal(s) in addition to nickel. For example, the nickelcontaining phase may contain an alloy of nickel and an additional metal,such as cobalt or copper. The metal phase is preferably finelydistributed in the ceramic phase, with an average grain size less than500 nanometers, such as 100 to 400 nanometers, to reduce the stressesinduced when nickel converts to nickel oxide. The ceramic phasepreferably comprises a doped ceria, such as a samaria, gadolinia oryttria doped ceria (in other words, the ceria may contain Sm, Gd and/orY dopant element which forms an oxide upon incorporation into theceria). Preferably, the doped ceria phase composition comprisesCe_((1-x))A_(x)O₂, where A comprises at least one of Sm, Gd, or Y, and xis greater than 0.1 but less than 0.4. For example, x may range from0.15 to 0.3 and may be equal to 0.2. Samaria doped ceria (SDC) ispreferred. Furthermore, the doped ceria may be non-stoichiometric, andcontain more than or less than two oxygen atoms for each one metal atom.Alternatively, the ceramic phase comprises a different mixed ionic andelectrically conductive phase, such as a perovskite ceramic phase, suchas (La, Sr)(Mn,Cr)O₃, which includes LSM, lanthanum strontium chromite,(La_(x)Sr_(1-x))(Mn_(y)Cr_(1-y))O₃ where 0.6≦x<0.9, 0.1<y<0.4, such asx=0.8, y=0.2, etc.

In one embodiment of the invention, the anode electrode contains lessnickel phase in a portion near the electrolyte than in a portion nearthe electrode surface distal from the electrode (i.e., the “free”electrode surface which faces away from the electrolyte). In anotherembodiment of the invention, the anode electrode contains less porosityin a portion near the electrolyte than in a portion near the “free”electrode surface distal from the electrode. In another embodiment ofthe invention, the anode electrode contains an additional metal, such asCo and/or Cu alloyed with the nickel, in a portion near the electrodesurface distal from the electrolyte. If desired, the additional metalmay be omitted from the portion of the anode near the electrolyte (i.e.,no intentionally introduced Cu or Co, but possible unintentionalbackground Co or Cu impurity or diffusion presence). Preferably, theanode electrode contains less nickel and less porosity in the portionnear the electrolyte and an additional metal in the portion distal tothe electrode.

FIG. 1 illustrates a solid oxide fuel cell (SOFC) 1 according to anembodiment of the invention. The cell 1 includes an anode electrode 3, asolid oxide electrolyte 5 and a cathode electrode 7. The electrolyte 5may comprise a stabilized zirconia, such as scandia stabilized zirconia(SSZ) or yttria stabilized zirconia (YSZ). Alternatively, theelectrolyte 5 may comprise another ionically conductive material, suchas a doped ceria. The cathode electrode 7 may comprise an electricallyconductive material, such as an electrically conductive perovskitematerial, such as lanthanum strontium manganite (LSM). Other conductiveperovskites, such as lanthanum strontium cobaltite (La,Sr)CoO₃,lanthanum strontium cobalt ferrite (La,Sr)(Co,Fe)O₃, etc., or metals,such as Pt, may also be used. In one embodiment, the cathode electrode 7is composed of several separately deposited sublayers, as will bediscussed in more detail below. This allows the cathode electrode to bemade thicker compared to single layer electrodes formed by prior artscreen printing method.

As shown in FIG. 1, the anode electrode 3 comprises a first portion 13and a second portion 23. The first portion 13 is located between theelectrolyte 5 and the second portion 23. As noted above, preferably, thefirst portion of the anode electrode 13 contains a lower ratio of thenickel containing phase to the ceramic phase than the second portion 23of the anode electrode. Furthermore, preferably, the first portion ofthe anode electrode 13 contains a lower porosity than the second portion23 of the anode electrode. In addition, the second portion 23 maycontain an additional metal alloyed with nickel, such as Co or Cu.Alternatively, the Cu or Co may be provided separately from Ni (e.g.,not pre-alloyed) into the anode electrode. Thus, the porosity and theratio of the nickel phase to the ceramic phase increases in as afunction of thickness of the anode electrode 3 in a direction from theelectrolyte 5 to the opposite surface of the anode electrode 3. Theadditional metal in the second portion is a step function. The firstportion contains no additional metal, while the second portion containsa uniform concentration.

For example, the first portion 13 of the anode electrode may contain aporosity of 5-30 volume percent and a nickel phase content of 1 to 20volume percent. The second portion 23 of the anode electrode may containa porosity of 31 to 60 volume percent, a nickel phase content of 21 to60 volume percent. The nickel containing phase may optionally containbetween 1 and 50 atomic percent, such as 5-30 at % of an additionalmetal, such as cobalt and/or copper, and the balance nickel.

In one embodiment, the first 13 and the second 23 portions of the anodeelectrode 3 comprise separate sublayers. Thus, the first region 13comprises a first sublayer in contact with the electrolyte 5 and thesecond region 23 comprises a second sublayer located over the firstsublayer 13. The first sublayer 13 contains a lower porosity and lowernickel to doped ceria ratio than the second sublayer 23. The secondsublayer 23 may contain an additional metal, such as Co or Cu, asdescribed above.

The first sublayer 13 may contain between 1 and 15 volume percent of thenickel containing phase, between 5 and 30 percent pores, such as between5 and 20 or between 15 and 25 volume percent pores, and remainder thedoped ceria phase. For example between 1 and 5 volume percent of thenickel containing phase, between 5 and 10 volume percent pores andremainder the doped ceria phase. The second sublayer 23 contains over 20volume percent nickel containing phase, between 20 and 60 volume percentpores, such as between 40 and 50 percent pores, and remainder is thedoped ceria phase. For example, it contains between 30 and 50 volumepercent of the nickel containing phase (which optionally contains 1-30at %, such as 5-10 at % Co and/or Cu and balance Ni), between 30 and 50volume percent pores and remainder the doped ceria phase. In the firstsublayer 13, the volume ratio of the nickel containing phase to thedoped ceria containing phase may range from 1:8 to 1:10, for example1:9. In the second sublayer 23, the volume ratio of the nickelcontaining phase to the doped ceria containing phase may range from 3:1to 5:1, for example 4:1. The first sublayer 13 may contain between 5 and25 weight percent nickel containing phase, such as between 10 and 20weight percent nickel containing phase, and between 75 and 95 weightpercent doped ceria containing phase, such as between 80 and 90 weightpercent doped ceria phase. The second sublayer 23 may contain between 60and 85 weight percent nickel containing phase, such as between 70 and 75weight percent nickel containing phase, and between 15 and 40 weightpercent doped ceria containing phase, such as between 25 and 30 weightpercent doped ceria phase. The optimum ratio of nickel to ceramic isdetermined by the requisite electronic conductivity, ionic conductivity,porosity, and electrocatalytic properties necessary for optimal anodeperformance.

Thus, the anode electrode 3 contains plurality of sublayers, eachvarying in composition, structure and nickel content. Each layer isapproximately 3-30 microns thick, such as 5-10 microns thick, forexample. Preferably, the first sublayer 13 is 3-6 microns thick and thesecond sublayer 23 is 6-10 microns thick for a total thickness of 9-16microns. The first sublayer in contact with the electrolyte has a higherdensity and lower nickel content than the one or more sublayers furtheraway from the electrolyte. A porosity gradient is established rangingfrom approximately 5-15% close to the electrolyte and increasing toabout 50% at the anode electrode's free surface. The nickel content inthe electrode increases in a similar manner as the porosity. The secondsublayer, farther away from the electrolyte, optionally has anadditional metal, such as Co or Cu, alloyed with nickel.

In another embodiment of the invention, each of the first 13 and second23 regions may comprise plural sublayers. For example, each region 13,23 may contain two sublayers, such that the anode electrode 3 contains atotal of four sublayers. In this case, the first region 13 comprises afirst sublayer in contact with the electrolyte and a second sublayerlocated over the first sublayer, while the second region 23 comprises athird sublayer located over the second sublayer and a fourth sublayerlocated over the third sublayer. In this configuration, a porosity ofthe anode electrode increases from the first sublayer to the fourthsublayer and the nickel phase content of the anode electrode increasesfrom the first sublayer to the fourth sublayer. In other words, thesublayer which contacts the electrolyte 5 has the lowest porosity andnickel phase content, while the sublayer which is located farthest fromthe electrolyte contains the highest porosity and nickel phase content(and the lowest doped ceria phase content).

For example, the first sublayer closest to the electrolyte 5 may containbetween 1 and 5 volume percent of the nickel containing phase, between 5and 15 volume percent pores and remainder the doped ceria phase. Thesecond sublayer may contain between 6 and 20 volume percent of thenickel containing phase, between 20 and 40 volume percent pores andremainder the doped ceria phase. The third sublayer may contain between25 and 35 volume percent of the nickel containing phase, between 30 and50 volume percent pores and remainder the doped ceria phase. The fourthsublayer which is farthest from the electrolyte 5 may contain between 35and 45 volume percent of the nickel containing phase (which optionallyincludes 1-30 at %, such as 5-10 at % Cu and/or Co and balance Ni),between 40 and 60 volume percent pores, and remainder the doped ceriaphase.

Fuel cell stacks are frequently built from a multiplicity of SOFC's 1 inthe form of planar elements, tubes, or other geometries. Fuel and airhas to be provided to the electrochemically active surface, which can belarge. As shown in FIG. 4, one component of a fuel cell stack is the socalled gas flow separator (referred to as a gas flow separator plate ina planar stack) 9 that separates the individual cells in the stack. Thegas flow separator plate separates fuel, such as a hydrocarbon fuel,flowing to the fuel electrode (i.e. anode 3) of one cell in the stackfrom oxidant, such as air, flowing to the air electrode (i.e. cathode 7)of an adjacent cell in the stack. The separator 9 contains gas flowpassages or channels 8 between the ribs 10. Frequently, the gas flowseparator plate 9 is also used as an interconnect which electricallyconnects the fuel electrode 3 of one cell to the air electrode 7 of theadjacent cell. In this case, the gas flow separator plate whichfunctions as an interconnect is made of or contains electricallyconductive material. An electrically conductive contact layer, such as anickel contact layer, may be provided between the anode electrode andthe interconnect. FIG. 4 shows that the lower SOFC 1 is located betweentwo gas separator plates 9.

Furthermore, while FIG. 4 shows that the stack comprises a plurality ofplanar or plate shaped fuel cells, the fuel cells may have otherconfigurations, such as tubular. Still further, while verticallyoriented stacks are shown in FIG. 4, the fuel cells may be stackedhorizontally or in any other suitable direction between vertical andhorizontal.

The term “fuel cell stack,” as used herein, means a plurality of stackedfuel cells which share a common fuel inlet and exhaust passages orrisers. The “fuel cell stack,” as used herein, includes a distinctelectrical entity which contains two end plates which are connected topower conditioning equipment and the power (i.e., electricity) output ofthe stack. Thus, in some configurations, the electrical power outputfrom such a distinct electrical entity may be separately controlled fromother stacks. The term “fuel cell stack” as used herein, also includes apart of the distinct electrical entity. For example, the stacks mayshare the same end plates. In this case, the stacks jointly comprise adistinct electrical entity, such as a column. In this case, theelectrical power output from both stacks cannot be separatelycontrolled.

A method of forming a planar, electrolyte supported SOFC 1 shown inFIGS. 1, 2 and 3 includes forming the cathode electrode 7 on a firstside of a planar solid oxide electrolyte 5 and forming the cermet anodeelectrode 3 on a second side of the planar solid oxide, electrolyte 5,such that a first portion of the anode electrode adjacent to theelectrolyte contains a lower porosity and a lower ratio of the nickelcontaining phase to the ceramic phase than the second portion of theanode electrode located distal from the electrolyte. The anode and thecathode may be formed in any order on the opposite sides of theelectrolyte.

The anode electrode containing a plurality of sublayers shown in FIG. 1may be formed by a screen printing method or by other suitable methods.For example, a first sublayer 13 containing a low porosity and a lownickel content can be screen printed on the electrolyte 5, followed byscreen printing a second sublayer 23 with a higher porosity and a highernickel content on the first sublayer 13. As described above, sublayer 13may be screen printed on the electrolyte 5, then dried at a temperaturebelow 150 C, such as about 70 C, followed by screen printing of sublayer23 over dried sublayer 13, optional drying of sublayer 23, and finallyfiring of both sublayers 13 and 23 at a temperature above 1000 C.

A non-limiting, exemplary method of making the anode and cathodeelectrodes by screen printing, drying and firing will now be described.

Bare, plate shaped solid oxide electrolytes 5, such as scandiastabilized zirconia electrolytes are unpackaged and placed into aslotted cassette 101. Each cassette is installed into an elevator 103which positions the individual electrolyte substrate 5 onto a walkingbeam conveyor 105. The walking beam conveyor transports the electrolytesubstrates 5 to a printing tool plate 107, while exposing the substratesto minimal abrasion during the process. A pick-up head 109 may be usedto place the electrolyte substrate from the end of the walking beamconveyor onto the tool plate. The pick up head may be configured with aBernoulli pad or vacuum pogo pin array to pick up the electrolytesubstrate.

The screen print cycle initiates once the pick up head 109 lowers theelectrolyte substrate 5 onto the tool plate 107. First, multiple snuggeralignment pins 111 collapse inward toward the substrate 5. The pins maybe fixed and/or pressure loaded. The combined inward movements positionthe substrate to a predetermined alignment position. A small amount oftool plate 107 vacuum is utilized to keep the substrate 5 fromoscillating between the snugger pins. Once the substrate is positioned,a hold down vacuum is applied and all snugger pins 111 retract out andaway from the work area. Afterward, the tool plate 107 carriage 113shuttles below a mesh screen 115 (e.g., a calendared mesh with a highwire density) and the print cycle initiates. The print cycle includesthe screen printing process using screen printing suitable ink, such asan ink with a relatively high solids loading of 80 to 93 weight percent,and screen tooling which defines the deposited ink image and inkdeposition characteristic.

Various process parameters may be adjusted to achieve a desired layerdeposition thickness and quality. Fine adjustments may be made byadjusting the print settings and course adjustments may be made byadjusting the screen configuration in the screen printing process.

In another embodiment, the ink may be dispensed on the screen before theprinting step, thus reducing the time the ink on the screen is exposedto external environment before drying. This increases the stability ofthe process and viscosity of the ink does not change due to drying. Thisprocess also reduces or eliminates contamination and reduces ink waste.

After the electrode screen print cycle is completed, the electrolytesubstrate 5 is lifted off the tool plate 107 either manually or by anysuitable machine or device, such as a pick up head 109. The tool platereturns to its home position to receive the next substrate. Thepreviously printed substrate 5 is transported down a conveyor 117 to apredetermined pick up location. Another pick up head 119, such as arobotic pick up head, lowers and surrounds the substrate with two ormore cleats. The cleats do not apply pressure to the substrate in orderto minimize chipping or damage to the substrate. The pick up head raisesand secures the substrate with the force of gravity. The head thentransports the electrolyte substrate to a dryer belt 121 and releasesthe substrate onto the dryer belt.

Any suitable dryer may be used. For example, the dryer 120 may includethe dryer conveyor belt 121, such as a woven stainless steel belt orother suitable conveyor belt travelling through an infrared heating zone123 heated by one or more infrared heating lamps 125. The electrolytesubstrate 5 is transported by the belt 121 to the heating zone 123 andheated in the heating zone by the heating lamp(s) 125. During theheating process, a percentage of the ink organics are released from theelectrode(s), which prepares the substrate for further processing.

The belt 121 may remain continuously moving while carrying the substratethrough the heating zone. Alternatively, the belt 121 may transport thesubstrate to the heating zone, then stop while the substrate is beingheated, followed by moving the substrate out of the heating zone aftercompletion of the heating. If desired, the dryer may comprise two ormore belts and/or two or more heating zones. In case of two or morebelts 121, the pick up head 119 may be pre-programmed or controlled byan operator or control system to sequentially place the substrates ondifferent belts to dry the substrates in parallel rather than in series.

The dried electrolyte 5 substrate is then removed from the dryer 120ether manually or by machine. Any suitable machine may be used. Forexample, a robotic Bernoulli pad or vacuum pick up head 127 withconfigured pogo pins may be positioned near the dryer exit. The pad orhead removes the substrate from the dryer belt and places it on awalking beam conveyor 129. The walking beam conveyor transports thesubstrate to an exit elevator 131, which then loads the substrate 5 intoa cassette 133 for further processing.

For electrodes comprising two or more sublayers, such as the anodeelectrode described above, these sublayers are printed separately andallowed to dry at a low temperature and then they are fired in a singleanode layer firing. Thus, as described above, after the first anodesublayer is printed on the electrolyte substrate and dried in the dryer,the electrolyte is returned from the dryer to the screen printingstation to deposit the second anode sublayer on the first anodesublayer. After the second anode sublayer is deposited on the firstsublayer and dried in the drier, the electrolyte substrate is providedfrom the dryer to a furnace where the plural anode sublayers are firedtogether in one firing step at a temperature that is at least threetimes higher than the drying temperature. For example, the drying stepsmay be conducted at a temperature of less than 150 C, such as 50 to 100C, for example 70 to 80 C. The firing may be conducted at a temperaturegreater than 1000 C, such as 1100 C to 1400 C.

In an alternative embodiment, the anode or cathode sublayers are printedupon each other without going through the drying process. In this“wet-on-wet” process, the first sublayer is screen printed on theelectrolyte, followed by screen printing the second sublayer on the wetfirst sublayer of the same electrode followed by firing, or drying andfiring the wet sublayers of this electrode. For example, two anodesublayers 13, 23 may be deposited on the electrolyte 5 without anintermediate drying step between the deposition of the first 13 and thesecond 23 anode sublayers. In this case, the first anode sublayer 13 isdeposited (e.g., screen printed) on the electrolyte, and while the firstsublayer is still wet, a second anode sublayer 23 is deposited (e.g.,screen printed) on the wet first sublayer 13. Both sublayers 13, 23 maythen be dried and fired, or fired without drying. Preferably, the firstsublayer 13 ink used during screen printing of the first sublayer 13 hasa higher viscosity than the second sublayer 23 ink used during screenprinting of the second sublayer 23. The first sublayer ink may have aviscosity that is 10% to 200% higher than the second sublayer inkdensity. This method increases process throughput, improves redoxbehavior of the anode, and eliminates a distinct boundary between thetwo sublayers. For example, the bottom portion of the anode 3 (i.e., theportion of anode 3 corresponding to sublayer 13 near the electrolyte 5)may have less nickel and more ceria phase compared to the top portion ofthe anode 3 (i.e., the portion of anode 3 corresponding to sublayer 23distal from the electrolyte 5). However, rather than having a distinctboundary between the sublayers 13, 23, a diffuse interface having anintermediate content of the nickel and ceria phases (i.e., having morenickel than in the bottom portion and less nickel than in the topportion of the anode) is located between the top and bottom portions ofthe anode.

As discussed above, after the printing and drying processes, eachsubstrate is subjected to a thermal process referred to as “firing”,which includes burnout and sintering of the electrolyte substrate. Ahigh temperature furnace may be used for the firing (i.e., both burnoutand sintering may be performed in the same furnace).

The electrolyte substrates may be loaded into the furnace in a differentmanner for anode and cathode electrode firing steps. For the electrodethat is fired first, the substrates may be stacked face to face into astack and then placed into the furnace. For the electrode that is firedsecond, each substrate may placed into the furnace without contactingadjacent substrates. For example, the substrates may be inserted intoone or more ceramic supports, frames or boats which are then insertedinto the furnace.

For example, for fuel cells in which the anode is deposited and firedfirst followed by depositing and firing the cathode, the followingprocess may be used. A plurality of planar (e.g., plate shaped) solidoxide electrolytes each have a first major side and a second major side.A first electrode is formed on the first major side of each of theplurality of planar solid oxide electrolytes. The first electrode may beformed by the dual screen printing method above or by any other suitablemethod. The plurality of solid oxide electrolytes are stacked into astack such that the first major sides containing the first electrode ofeach pair of adjacent electrolytes in the stack face each other. Thus,except for a top and bottom electrolyte in the stack, the second majorsides of each pair of adjacent electrolytes in the stack face the secondmajor side of an adjacent electrolyte in the stack. The stack is thenfired. Preferably, the second major sides of each electrolyte in thestack lack an electrode during the step of firing, and the first majorsides containing the first electrode of each pair of adjacentelectrolytes in the stack contact each other during the step of firing.

For example, for anode electrode firing, the fuel cells (e.g., theelectrolyte substrates with printed anodes) may be stacked in contactwith each other into a stack. The cells may be oriented face-to-face inan alternating fashion. In other words, the anode electrodes of a pairof adjacent cells are placed in contact with each other, and theunprinted cathode sides of two other adjacent cells contact theunprinted cathode sides of the pair of cells. Thus, the anode printedelectrolyte substrates are placed with the printed surfaces facing eachother. Subsequent substrates are stacked in identical fashion to createa “pack” or stack of substrates which are processed together as a unit.Each pack is placed on a shelf. A weight, such as a ceramic block or alid, is placed on top to create a condition referred to as constrainedsintering. Once a shelf is fully populated, subsequent ceramic block andshelves are placed vertically creating a second tier. Once the secondtier is populated, subsequent tiers are built to the capacity of thefurnace. The furnace firing program (i.e., temperature-time schedule) isthen initiated which completes the anode firing process. Because a largestack of cells may be fired in this way, furnace throughput is kepthigh. The cell construction with anode and cathode electrodes formedwith two different ink formulations and printing conditions allowsstacking two-layer printed substrates face-to-face in pairs and preventsbonding during sintering.

Preferably, the anode and the cathode layer firing steps are separatefiring steps. Thus, after the anode sublayers deposited, dried and thenfired together, the electrolyte substrate is turned upside down andreturned to the printing station. At the printing station, one or morecathode sublayers are deposited, dried and then fired together as willbe described below.

For cathode electrode firing, the cells (e.g., electrolyte substrates,preferably with fired anodes printed on one side and unfired cathodesprinted on the other side) are held in ceramic supports, frames or boats(referred to as “fixtures”). These ceramic fixtures hold each cell inplace and spaced apart from adjacent cells. The fixtures prevent grossdeformation of the cell during firing in order to avoid inducing camberinto the part. The ceramic fixtures may be made from high purity alumina(e.g., 99.9% pure) or other suitable materials.

In another embodiment, the anode and the cathode layer firing isperformed in the same step. In this case, the electrolyte substratecontaining dried anode and cathode electrode layers is provided into afurnace and the anode and cathode are co-fired during the same step. Theco-firing compensates for the camber of the cell which occurs when eachside is sintered individually. The co-firing may be conducted with cellswhich are printed on both sides to be stacks in contact with each other(e.g., an anode of a first cell contacting anode of an adjacent secondcell, and a cathode of the first cell contacting cathode of an adjacentthird cell) without using ceramic fixtures for supporting eachindividual cell and separating adjacent cells from each other.

In another alternative embodiment, the cathode electrodes are notsintered at all prior to being placed into a fuel cell stack. In thisembodiment, the anode layer or sublayers are printed on the electrolyte,dried and sintered. Then the cathode layer or sublayers are printed onthe electrolyte and dried, but not sintered to complete one solid oxidefuel cell. The solid oxide fuel cell with the dried but unsinteredcathode is then placed into a solid oxide fuel cell stack where adjacentfuel cells are separated by conductive (e.g., metal) interconnects/gasseparator plates and sealed by glass or glass ceramic seals. The wholestack is then sintered to sinter the glass or glass ceramic seals. Thesintering of the cathode electrodes in all cells of the stack occurs atthe same time as when the seals in the stack are sintered. Thus, in thisembodiment, both seals and the unfired and unsintered cathode electrodesin a solid oxide fuel cell stack (which contains SOFCs andinterconnects) are sintered in the same step.

The furnace firing temperature setting may include a first time periodor step during which the furnace is kept at steady temperature ramprate, slowly increasing temperature or slowly decreasing temperature atthe temperature were binders begin to burn-out from the printed anddried inks. The length of this first step is such that binders arecompletely burned out. The furnace temperature is then increased to asecond, higher sintering temperature for a second time period or stepwhere the electrode sintering occurs. The firing temperature profile maycomprise heating the substrate at a first temperature for a sufficienttime to burn out the electrode ink binders followed by heating thesubstrate at a second temperature higher than the first temperature fora sufficient time period to sinter the electrode. The firing may beconducted at a temperature greater than 1000 C, such as 1100 C to 1400C. The anode and cathode electrode layers may be burned out at 1100 to1200 C and sintered at temperatures below 1300 C, such as 1200 to 1300C, thus reducing the cost of furnaces.

Preferably, convective furnace processing is utilized during anode andcathode firing in order to ensure complete binder burn-out. Convectivefurnace processing ensures there is sufficient air flow to oxidize andclear the binder off-gassed species. In an alternative embodiment,oxygen enriched air (i.e., oxygen added to air such that the ambientcontains a higher concentration of oxygen than atmospheric air), pureoxygen or other active, oxidizing atmosphere may be utilized during thebinder burn-out stage of anode and/or cathode firing in order to improvethe process effectiveness and throughput. The sintering stage of thefiring may be conducted in atmospheric air or an inert ambient (e.g.,nitrogen ambient). Thus, in this embodiment, the first burn out firingstep or stage is conducted in an ambient that contains a higher oxygencontent than the second sintering firing step or stage.

The fabrication of the anode with a Ni—Co alloy can be achieved by firstsynthesizing a Ni_(1-x)Co_(x)O powder with the desired stoichiometry andmixing it with a desired amount of SDC powder. For example, 0.05≦x≦0.3.Any suitable ink processing (i.e., mixing the powders with the inksolvent), ink deposition, such as screen printing described above, andfiring/sintering steps, such as the ones described above, can be usedfor anode fabrication. During anode reduction, the Ni_(1-x)Co_(x)O/SDCcomposite reduces to Ni—Co/SDC cermet and porosity. Alternatively, theNi—Co alloy (i.e., a metal alloy rather than a metal oxide) can beproduced in powder form, mixed with the SDC, deposited, and sintered ina reducing atmosphere to produce a similar anode compositionally with adifferent microstructure. Other alloying elements, such as Cu, may beused instead of or in addition to Co. Likewise, other ceramic materials,such as GDC, etc., may be used instead of or in addition to SDC.

FIGS. 5A and 5B depict Scanning Electron Microscopy images of a SOFCoriented from cathode at the top of the image to anode at the bottom.The topmost layer in the image is the cathode electrode 7. The nextlayer is the electrolyte layer 5. The bottom layer is the anode 3. Theanode is comprised of two sublayers 13 and 23. The SEM images show thecondition of the anode sublayers after operation under conditions offuel starvation using non-reformed hydrocarbon fuel.

FIG. 5A shows an image of a SOFC made according to a comparativeexample. The SOFC of the comparative example is described in U.S.application Ser. No. 11/907,204 filed on Oct. 10, 2007 and incorporatedherein by reference in entirety. The SOFC of the comparative examplelacks the Cu or Co in the upper sublayer 23. SOFC of the comparativeexample exhibits adequate performance. However, the image shows evidenceof some of cracking.

FIG. 5B shows an image of a SOFC made according to an example ofinvention. The anode of the invention is thinner, e.g., about 9-16microns, than the about 30 micron thick anode of the comparativeexample. The anode also contains cobalt substituting nickel in the uppersublayer. The anode of the example of the invention shows no evidence ofstructural or mechanical failure, such as cracking, delamination ordusting. Without wishing to be bound by a particular theory, theinventors attribute this change to the addition of Co to the nickelphase, forming a Ni—Co alloy. The decreased Ni concentration is believedto decrease the catalytic activity on the surface of the anode resultingin the spreading of the reformation reaction across the surface of theanode rather than having the reformation reaction be limited to theleading edge of the anode. Because the reaction is dispersed, thethermo-mechanical stresses are significantly decreased. Therefore, theanode of the example of the invention suffers less or no damage comparedto the anode of the comparative example.

FIG. 6 shows the results of mechanical load testing. The testingconsisted of four point bend test of cells with the anode only. Thegraph plots the results for the comparative example (line “COMP”) aswell as the example of the invention (line “INV”). The plot of theresults of the comparative example reveals a mean mechanical load of1.369 kgf with a standard deviation of 0.16943. The plot of the datafrom the example of the invention shows a mean mechanical load of 1.435kgf with a standard deviation is 0.06952. While both types of cellsshowed acceptable results, the cells of the example of the inventionshowed results which were consistent every time in their failure load,while the comparative example cells showed many outliers and generallyinconsistent behavior. The cells of the example of the invention have ahigher mean load to failure as well as the consistency, which isindicative that the interface between the electrolyte and electrode isbetter than in the comparative example cells. This may be attributed toless or no cracking of the anode of the example of the invention.

FIG. 7 depicts a plot of the median voltage compliance results at 30 ampoperation for several fuel cell stacks according to the comparativeexample and the example of the invention. One stack tested containedsome cell made according to the comparative example and other cells madeaccording to the example of the invention. The data characterizes outputvoltage potential versus temperature for the stacks at beginning of lifeunder operating conditions. Output potential was measured as temperaturewas varied from 770° C. to 850° C., with a steam to methane (e.g.,carbon) ratio of 1.9, and fuel utilization of 90%. The results showhigher output potential for the invention (such as an about 60 mV higherpotential), thus providing an improved result in comparison to thesufficient results of the comparative example. The comparative exampledata at 770° C. indicates evidence of some coking, which is not observedfor the example of the invention at the same temperature.

FIG. 8 is a graph of output voltage for a 25 cell stack containing cellsaccording to both the comparative example and the example of theinvention after 200 hours of operation at fuel utilization of 75% andoperating at 850° C. The circled data points indicate the output voltageresults for cells of the example of the invention after 200 hours ofoperation. The un-circled data points indicate the output voltageresults for cells of the comparative example after 200 hours ofoperation. Of note is the cells of the invention all show a lower degreeof degradation of output potential after operation than the stacks of acomparative example.

FIG. 9 is a graph of methane conversion versus mass flow rate of naturalgas (in standard liters per minute) for pure nickel of the comparativeexample and for a Ni—Co alloy (10 atomic percent Co, 90 atomic percentNi) of the example of the invention at 750 and 800° C. As can be seenfrom the figure, the electrocatalytic reformation conversion of pure Niis higher compared to a Ni—Co alloy where the volume fraction of metaland ceramic are identical for the two second layers 23. The higher themass flow, the more efficient the catalyst has to be to reform all themethane. The Ni—Co catalyst provides a slower catalytic reaction becausemethane is detected at a lower mass flow rate.

The anode electrode contains a doped ceria phase rich interface at athree phase boundary with the electrolyte and a rich nickel phase regionadjacent to the “free” surface of the anode electrode which is distalfrom the electrolyte (i.e., the surface of the anode 3 which faces theinterconnect 9). Without wishing to be bound by a particular theory, thepresent inventors believe that the greater stability of the anodeelectrodes of the embodiments of the present invention under conditionsof very high fuel utilization can be primarily attributed to thepresence of the ceria rich interface at the three phase boundary. Themixed ionic and electronic conducting nature of the doped ceria acts asa buffer to the oxygen ion flux through the electrolyte, thus mitigatingthe rapid conversion of nickel to nickel oxide. Mechanical damage of theelectrode/electrolyte is avoided and upon the establishment of normaloperating conditions, minimal polarization change in the anode isobserved. Because the ceria-based ceramic has a lower electronicconductivity than nickel, the presence of a small amount of nickelimproves the conductivity of the first sublayer(s) without causing anydeleterious effect to the mechanical stability under fuel starvationconditions.

The anode electrode further contains a metal rich region of a nickelalloy distal from the electrolyte. The inventors believe that decreasingthe nickel concentration will disperse the steam reforming active regionof the anode away from the leading edge. Because nickel is such a strongelectrocatalyst, it is believed the high Ni concentration results in amajority of the reforming occurring within a few centimeters of theanode's length (i.e., at the leading edge where the hydrocarbon fuelenters the anode space between the anode and the interconnect). Theelevated concentration of highly endothermic steam reforming results inthermo-mechanical stress and anode delamination. Partially deactivatingthe Ni catalyst by replacing it with a less catalytic or non-catalyticmetal, such as Cu and/or Co in the entire upper anode sublayer, allowsthe reforming reaction to be spread over the entire length (i.e., area)of the anode surface from fuel inlet to outlet, and decreases the hightemperature gradient. The decreased temperature gradient results inlower thermo-mechanical stress at the leading edge of the cell therebyminimizing the cause of the anode delamination and failure mechanism.This also lowers the dusting and Ni carbide formation. Furthermore,since the doped ceria ceramic phase of the anode, such as SDC, iselectrocatalytically active, the total catalytic activity of the anodeis not significantly reduced.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedescription was chosen in order to explain the principles of theinvention and its practical application. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

The invention claimed is:
 1. A method of making a solid oxide fuel cell(SOFC), comprising: forming a first sublayer of a first electrode on afirst side of a planar solid oxide electrolyte; drying the firstsublayer of the first electrode; forming a second sublayer of the firstelectrode on the dried first sublayer of the first electrode prior tofiring the first sublayer of the first electrode; firing the first andsecond sublayers of the first electrode during a same first firing step;and forming a second electrode on a second side of the solid oxideelectrolyte, wherein the same first firing step comprises placing astack of solid oxide electrolytes into a furnace such that surfaces ofthe electrolytes printed with the first electrode face and contact eachother and convectively firing the first electrode.
 2. The method ofclaim 1, wherein the step drying is conducted at a first temperature,the step of firing is conducted at a second temperature, and the firsttemperature is at least three times less than the second temperature. 3.The method of claim 2, wherein the first temperature is less than 150 Cand the second temperature is greater than 1000 C.
 4. The method ofclaim 1, wherein the first electrode comprises an anode electrode. 5.The method of claim 4, wherein: the first sublayer of the anodeelectrode comprises a cermet comprising a nickel containing phase and adoped ceria containing phase; the second sublayer of the anode electrodecomprises a cermet comprising a nickel containing phase and a dopedceria containing phase; and the first sublayer of the anode electrodecontains a lower ratio of the nickel containing phase to the ceramicphase than the second sublayer of the anode electrode.
 6. The method ofclaim 5, wherein the doped ceria phase comprises a Sm doped ceria phaseand the nickel containing phase comprises nickel or 1 to 50 atomicpercent of at least one of Co or Cu and balance nickel.
 7. The method ofclaim 1, wherein the first electrode comprises a cathode electrode. 8.The method of claim 1, wherein the step of forming the second electrodecomprises: forming a first sublayer of a second electrode on the secondside of the planar solid oxide electrolyte; drying the first sublayer ofthe second electrode; forming a second sublayer of the second electrodeon the dried first sublayer of the second electrode prior to firing thefirst sublayer of the second electrode; and firing the first and secondsublayers of the second electrode.
 9. The method of claim 8, wherein thestep of firing the first and second sublayers of the second electrodecomprises a second firing step which occurs after the same first firingstep.
 10. The method of claim 8, wherein the step of firing the firstand second sublayers of the second electrode comprises the same firstfiring step such that the first and the second electrodes are firedduring the same first firing step.
 11. The method of claim 1, furthercomprising drying the second sublayer of the first electrode prior tothe same first firing step.
 12. The method of claim 1, wherein: the stepof forming the first sublayer of the first electrode comprises screenprinting the first sublayer; and the step of forming the second sublayerof the first electrode comprises screen printing the second sublayer.13. The method of claim 12, wherein the step of forming the firstsublayer of the first electrode comprises: placing the electrolyte intoa slotted cassette; installing the cassette into an elevator whichpositions the electrolyte onto conveyor; transporting the electrolyte toa screen printing tool plate; aligning the electrolyte into position onthe screen printing tool plate; applying a hold down vacuum to hold theelectrolyte on the screen printing tool plate; and screen printing thefirst sublayer of the first electrode using an ink comprising 80 to 93%by weight solids.
 14. The method of claim 13, wherein the step of dryingthe first sublayer of the first electrode comprises placing theelectrolyte on a belt, transporting the electrolyte to a heating zone,and drying the first sublayer of the first electrode using a infraredheating lamp.
 15. The method of claim 1, wherein: the same first firingstep comprises a burnout stage conducted in a first ambient at a firsttemperature and a sintering stage conducted in a second ambient at asecond temperature; the second temperature is higher than the firsttemperature; and the first ambient contains a higher oxygen content thanthe second ambient.
 16. The method of claim 1, wherein the step offorming the second electrode comprises screen printing the secondelectrode on the second side of the planar solid oxide electrolyte,drying the second electrode, and firing the second electrode in a secondfiring step.
 17. The method of claim 16, wherein the second firing stepcomprises inserting the electrolyte containing the dried secondelectrode into a ceramic fixture, placing the fixture into the furnacesuch that the electrolyte does not contact adjacent electrolytes, andfiring the second electrode.
 18. The method of claim 1, wherein: thefirst electrode comprises an anode electrode; and the second electrodecomprises a cathode electrode.
 19. The method of claim 18, wherein thestep of forming the second electrode comprises: printing the cathodeelectrode on the second side of the solid oxide electrolyte after firingthe first and second sublayers of the anode electrode during the samefirst firing step; drying the cathode electrode; placing the electrolytecomprising the fired anode electrode and the dried, unfired cathodeelectrode into a solid oxide fuel cell stack; forming glass or glassceramic seals in the solid oxide fuel cell stack; and sintering thesolid oxide fuel cell stack to sinter the glass or glass ceramic sealsand to sinter the cathode electrode in the same step.
 20. A method ofmaking solid oxide fuel cells (SOFCs), comprising: providing a pluralityof planar solid oxide electrolytes, each electrolyte having a firstmajor side and a second major side; forming a first electrode on thefirst major side of each of the plurality of planar solid oxideelectrolytes; stacking the plurality of solid oxide electrolytes into astack such that the first major sides containing the first electrode ofeach pair of adjacent electrolytes in the stack face each other; andfiring the stack, wherein: the first major sides containing the firstelectrode of each pair of adjacent electrolytes in the stack contacteach other during the step of firing.
 21. The method of claim 20,wherein: the second major sides of each electrolyte in the stack lack anelectrode during the step of firing.
 22. The method of claim 21, whereinexcept for a top and bottom electrolyte in the stack, the second majorsides of each pair of adjacent electrolytes in the stack face the secondmajor side of an adjacent electrolyte in the stack.
 23. The method ofclaim 21, further comprising: drying the first electrode after the stepof forming and before the step of firing; and forming a second electrodeon the second major side of each of the plurality of planar solid oxideelectrolytes after the step of firing.
 24. The method of claim 20,wherein: the first electrode comprises an anode electrode; the step offorming the first electrode comprises screen printing a first sublayerof the first electrode, drying the first sublayer, and screen printing asecond sublayer of the first electrode on the first sublayer; and thestep of firing comprises placing the stack into a furnace, placing aweight on the stack and convectively firing the first electrode.
 25. Amethod of making a solid oxide fuel cell (SOFC), comprising: forming afirst sublayer of a first electrode on a first side of a planar solidoxide electrolyte; drying the first sublayer of the first electrode;forming a second sublayer of the first electrode on the dried firstsublayer of the first electrode prior to firing the first sublayer ofthe first electrode; firing the first and second sublayers of the firstelectrode during a same first firing step; and forming a secondelectrode on a second side of the solid oxide electrolyte; wherein: thefirst electrode comprises an anode electrode; and the second electrodecomprises a cathode electrode; wherein the step of forming the secondelectrode comprises: printing the cathode electrode on the second sideof the solid oxide electrolyte after firing the first and secondsublayers of the anode electrode during the same first firing step;drying the cathode electrode; placing the electrolyte comprising thefired anode electrode and the dried, unfired cathode electrode into asolid oxide fuel cell stack; forming glass or glass ceramic seals in thesolid oxide fuel cell stack; and sintering the solid oxide fuel cellstack to sinter the glass or glass ceramic seals and to sinter thecathode electrode in the same step.